1. Field of the Invention
The present invention relates to an optical system for endoscopes to be used for observing the interior of pipes, having an imaging optical system and a reflecting mirror arranged in front of (i.e., on the object side of) the imaging optical system, chiefly used to view an inner wall of a pipe laid under the ground and the like.
2. Description of the Prior Art
A conventional optical system for endoscopes of the type, as shown in FIG. 1 by way of example, has fundamentally comprised an image guide fiber bundle 2 arranged in an endoscope body 1, an imaging lens unit 3 arranged in front of the image guide 2, a conical mirror 4 disposed in front of the imaging lens unit 3 in such a way that its optical axis coincides with that of the imaging lens unit 3, and an aperture stop 5 provided between the imaging lens unit 3 and the conical mirror 4 so that light coming from an inner pipe wall S, after being reflected from the conical mirror 4, traverses the imaging lens unit 3, thereby being formed on an entrance end face of the image guide 2 as an image of the inner pipe wall S (German Utility Model No. 7834180). In this conventional instance, however, the problems have been encountered that intensive negative distortion of the imaging lens unit 3 diminishes magnification in a direction parallel with the axis of the pipe in the periphery of a visual field and consequently makes it difficult to observe delicate appearance of the inner pipe wall S.
Thus, to settle the preceding problems, there have been proposed an optical system including the conical mirror 4 configured so that its generatrix assumes a concave curve as depicted in, for example, FIG. 2 (Japanese Patent Preliminary Publication No. Sho 60-42728) and an optical system including the conical mirror 4 configured so that, as shown in FIG. 3, a curved surface 7 is attained by rotating an arc segment drawn, with a center G at a position shifted from an optical axis 6 in a plane including the optical axis 6, about the optical axis, on the opposite side of the center G in relation to the optical axis 6 (Japanese Utility Model Preliminary Publication No. Sho 62-113317).
However, the above two conventional instances have each caused the problems that curvature of the reflecting surface of the conical mirror 4 in a direction of a spherical segment (i.e., a direction of the circumference of a section cut by a plane normal to the optical axis) is largely different from that in a meridional direction (i.e., a direction of the generatrix), with the result that imaging positions in the direction of the spherical segment and the meridional direction are considerably shifted from each other, that is, an optical system which is large in astigmatic difference is brought about. For this reason, it has been required that the diameter of the aperture stop 5 is reduced or the size of the conical mirror 4 is increased to thereby diminish the difference in curvature between the direction of the spherical segment and the meridional direction. As a result, there have been further problems that a dark optical system with large F-number (F No.) is caused or the possibilities of the miniaturization and the reduction to a fine diameter of the distal end portion of the endoscope are made difficult.